1. Technical Field
The present invention relates to a multi-function light sensor.
2. Related Art
An illumination intensity sensor is a light sensor that detects illumination. Use can be made thereof, for example, for adjusting the brightness of a display, or switching light sources on or off, according to the peripheral illumination. Spectral sensitivity characteristics close to human spectral sensitivity is therefore required for illumination intensity sensors. Human spectral sensitivity can be represented as a relative spectral sensitivity curve having a peak between wavelengths of 500 nm and 600 nm.
Silicon photodiodes are most generally used for light receiving elements. The spectral sensitivity of silicon photodiodes generally has a peak at wavelengths of 600 nm to 700 nm, with sensitivity spreading out in a wide distribution to wavelengths from 300 nm to 1100 nm. Therefore when silicon photodiodes are used as illumination intensity sensors spectral sensitivity correction must be performed.
For example, one method of spectral sensitivity correction is, as shown in FIGS. 20A and 20B, to prepare a standard silicon photodiode “PD1” and a photodiode with sensitivity on the infrared side “PD2”, and to take the difference between the respective photoreceptor currents. Spectral sensitivity characteristics can be obtained that are close to human spectral sensitivity by calculating the difference of each of the photoreceptor currents, as shown in FIG. 20C.
As a method of element isolation (separation) between adjacent light receiving elements, a method is proposed in Japanese Patent Application Laid-Open (JP-A) No. 2001-352094 to prevent occurrence of cross-talk due to leak current by forming a trench groove for element isolation after forming pn junctions. In order to address the same objective JP-A No. 2004-104084 proposes a configuration with a circuit interposed between an AF photodiode and an AE photodiode in a camera Auto Focus (AF) sensor installed with an Auto Exposure (AE) function.
In order to develop a multi-function light sensor at low cost, the present inventor has invented a light sensor device equipped with both an ultraviolet light sensor and a visible light sensor. An example of such a device is shown in FIGS. 21A and 21B. As shown in the figures, this device has a light receiving element “PD1” and a light receiving element “PD2” formed adjacent to each other on a silicon substrate as visible light sensors. As an ultraviolet sensor, a light receiving element “UV-PD” is formed in a semiconductor layer that is formed on the silicon substrate with an insulating film, such as a silicon oxide film etc., interposed between the silicon substrate and the semiconductor layer.
Namely, the ultraviolet light sensor is formed on a semiconductor layer of a so-called SOI structure. A light sensor device equipped with both an ultraviolet light sensor and visible light sensors can be obtained by fabricating the light receiving element PD1 and light receiving element PD2 on the same silicon substrate as the light receiving element UV-PD, with an insulating film interposed in this manner therebetween.
There is a need to simplify device structure and to suppress the chip surface area to a small amount, in particular when considering providing the above light sensor device at low cost. There is room for improvement with respect to these points in the light sensor device shown in FIGS. 21A and 21B. For example, the light receiving element PD1 and the light receiving element PD2 are formed adjacent to each other on the silicon substrate, however when the light receiving element PD1 and the light receiving element PD2 are formed adjacently in order to reduce the chip surface area, a problem arises in that a leak current flows between the adjacent light receiving element PD1 and light receiving element PD2 due to the silicon substrate being a semiconductor.
When a leak current flows between adjacent photodiodes, then dark current when there is no incident light cannot be measured. Namely, since the leak current flows irrespective of whether or not there is incident light, the light sensor becomes unable to fulfill its role of light detection. In practice therefore, a multi-function light sensor cannot be provided at low cost.
However, a structure in which a trench groove is formed in the silicon substrate, as in JP-A No. 2001-352094, cannot be adopted due to the limitation that the fabricating process would become complicated. In addition, a structure in which a transistor is disposed on the silicon substrate between the light receiving element PD1 and the light receiving element PD2 could be considered, as in JP-A No. 2004-104084, however, even if leak current between the light receiving element PD1 and the light receiving element PD2 is stopped, there is concern that leak current would occur between the transistor and the light receiving elements.